Dr. Tran reports no financial relationships relevant to this field of study.

SYNOPSIS: An analysis of a large ICU database showed that hourly monitoring of urine output was associated with improved detection of acute kidney injury (AKI) and lower 30-day mortality in patients with AKI.

Acute kidney injury (AKI) is common in the ICU and is associated with increased morbidity and mortality.1 Although guidelines define AKI by both serum creatinine (SC) and urine output (UO) criteria, monitoring standards, especially regarding UO, vary widely, and it is unclear whether intensive monitoring of SC and UO are associated with improved outcomes.

Jin et al analyzed data from 15,724 patients after applying several exclusion criteria, including history of chronic dialysis, renal transplant, baseline SC > 4, ICU stay, or death within 48 hours of ICU admission, and no vasopressor support or mechanical ventilation in 24 hours from ICU admission, to a large database of patients admitted to any one of eight ICUs at the University of Pittsburgh. Overall, 4,049 patients received intensive UO monitoring, defined as hourly recordings with no gaps > 3 hours in the initial 48 hours of ICU admission. A total of 12,156 patients received intensive SC monitoring, defined as three calendar days of at least one measurement of SC after ICU admission. AKI was defined according to the maximum Kidney Disease Improving Global Outcomes (KDIGO) criteria within seven days of ICU admission using both SC and UO criteria and considered moderate-severe as defined by KDIGO stage 2-3. Baseline characteristics were similar between all groups that underwent intensive vs. less intensive monitoring of SC and UO stratified by stage 2-3 AKI. Intensive monitoring of UO was associated with higher rates of AKI (odds ratio [OR], 1.22; 95% confidence interval [CI], 1.11-1.35; P < 0.001) as opposed to intensive monitoring of SC (OR, 1.11; 95% CI, 1.00-1.24; P = 0.05). In patients with AKI, intensive monitoring of UO was associated with improved 30-day mortality compared to patients with less intensive UO monitoring (hazard ratio, 0.85; 95% CI, 0.77-0.94; P = 0.001). This was not seen in the subgroup of patients without AKI. In contrast, there was no significant association between intensive monitoring of SC and survival in patients with AKI.

COMMENTARY

Findings from this study argue for closer monitoring of UO for critically ill patients who, because of their clinical status, are at high risk for developing AKI. Although hourly UO is emphasized often in surgical ICUs, monitoring may be less standardized in medical ICUs, especially given the push toward removing Foley catheters early (or avoiding them altogether) to avoid hospital-acquired urinary tract infections.

The observation that AKI was found more frequently in patients with intensive UO monitoring is not surprising, as the addition of UO criteria will increase the sensitivity of KDIGO guidelines in detecting patients with AKI. In fact, other studies have reported a higher incidence of AKI using UO compared to SC criteria and the possibility of “subclinical” AKI when patients with parenchymal kidney abnormalities on biopsy do not meet KDIGO creatinine criteria for AKI, suggesting there may be a delay in creatinine rising.2,3 However, because this is a database study, we cannot assume a direct cause-and-effect relationship between intensive UO monitoring and improved 30-day mortality. Although baseline ICU characteristics were mostly similar, the intensive UO monitoring group appeared less critical given less vasopressor use and less fluid given, although it is unclear if the latter finding was because they received intensive UO monitoring, which made it easier to titrate fluids. In addition, it is unknown what specific interventions may have led to the improved outcomes seen in the intensive UO monitoring group. Was it because they demonstrated a lower net fluid balance (higher cumulative fluid balance is associated with higher mortality4), because they were less likely to receive nephrotoxic agents as a result of early recognition of AKI, or because they were initiated on renal replacement therapy earlier with better outcomes, as seen in the ELAIN trial?5

We are unable to answer these questions because of the limitations of this database study. Intensive monitoring of UO in the ICU likely is easy enough from a nursing perspective if the patient is wearing an indwelling catheter, although the risks/benefits of keeping urinary catheters in place for UO monitoring vs. rates of catheter-associated urinary tract infections will need to be weighed carefully.